Whether paying an employee a lump sum (a salary) or by the hour is just a method of payment. The method of payment does not determine whether the employer must pay the employee overtime.
The state and federal governments determine which employees must be paid overtime and when overtime must be paid. If an employee is not exempt from being paid overtime, the employer does not get out of paying overtime by merely changing the method of payment or by "agreeing" with the employee not to pay overtime.
Most employers in WA are regulated by WA Labor & Industries. Your employer likely is regulated by L&I.
You can review the information provided by L&I to see whether you are entitled to overtime pay under WA laws: http://www.lni.wa.gov/WorkplaceRights/Wages/Overtime/default.asp .
If you are not legally entitled to be paid overtime, your employer can require you to work more than 40 hours per workweek unless your industry is one of those (such as truck driving) whose hours are regulated.
The law is also very flexible in when an employer can schedule to work. Unless you have a religious reason to not work on Saturdays, your employer likely can require you to work on Saturdays unless the employer is unlawfully discriminating against you.
L&I has a lot of information at its website. If you still think the employer is wrong after reviewing the information, you should review your specific facts with your attorney to find out your legal options.
Filing a complaint directly with L&I is free. If your employer is regulated by L&I and L&I thinks your employer is violating the laws, L&I will investigate the employer and fine it if L&I finds the employer violates the laws.